Power requirements for modern, solid state electronics are progressively becoming lower and lower. For example, the Liquid Crystal Display (LCD) may require only milliwatts for operation, and the field effect transistor (FET) can respond to even small static charges. This has enhanced the utility of wireless power transmission as an approach for energizing electronics. An example and application of wireless power for electronics is the Radio Frequency Identification (RFID) transponder “tag”, which can allow a method of storing and remotely retrieving data to a reader.
As background, wireless power transmission can be the conveyance of electrical energy by radio frequency (RF) techniques, such as the electric power transmitted and received between two radio antennas. Depending on antenna size and range of transmission, the energy may convey by far fields or by near fields, and the energy transferred weak or small. Although it may be impractical or even hazardous to convey high power over great distances, wireless power transmission can be effective, safe and reliable for lower powers and shorter ranges. Generally, the shorter the range the greater the power that can be conveyed. There is a need for wireless power that is more easily integrated with communications.
It is possible to have dual linear or dual circular polarization channel diversity. That is, a frequency may be reused if one channel is vertically polarized and the other horizontally polarized. Or, a frequency can also be reused if one channel uses right hand circular polarization (RHCP) and the other left hand circular polarization (LHCP). Polarization refers to the orientation of the E field in the radiated wave, and if the E field vector rotates in time, the wave is then said to be rotationally or circularly polarized. Orthogonal polarizations, e.g. polarizations that are perpendicular, can be vertical linear and horizontal linear or right and left hand circular, and they can be uncoupled as separate channels in communications.
The dipole antenna has been perhaps the most widely used of all the antenna types. It is of course possible however to radiate from a conductor which is not constructed in a straight line. Preferred antenna shapes are often Euclidian, being simple geometric shapes known through the ages. In general, antennas may be classified as to the divergence or curl of electric current, corresponding to dipoles and loops, and line and circle structures.
Many structures are described as loop antennas, but standard accepted, e.g. canonical, loop antennas are a circle. The resonant loop is a full wave circumference circular conductor, often called a “full wave loop”. The typical prior art full wave loop is linearly polarized, having a radiation pattern that is a two petal rose, with two opposed lobes normal to the loop plane, and a gain of about 3.6 dBi. Reflectors are often used with the full wave loop antenna to obtain a unidirectional pattern.
Dual linear polarization (simultaneous vertical and horizontal polarization from the same antenna) has commonly been obtained from crossed dipole antennas. For instance, U.S. Pat. No. 1,892,221, to Runge, proposes a crossed dipole system. A dual polarized loop antenna could be more desirable however, as loops provide greater gain in smaller area. An approach to dual circular polarization in single loops is described in U.S. Published Patent Application No. 20080136720, to Parsche et. al.
U.S. Pat. No. 645,576, to Tesla, is directed to wireless power transmission. A pair of “elevated terminals” function as monopole antennas to accomplish radiation and reception of electric energy by radio. Spiral loading inductors were included to force antenna resonance. At ranges beyond λ/2π, operation may have been by far field radiation of electromagnetic waves, and at ranges less than λ/2π, the antennas radial reactive electric field (near E field) may have allowed for additional coupling. The spiral loading inductors were collocated with other windings to form a transformer in situ, to couple the generators and loads to the antennas. Connections were not however provided, to include a separate communications channel along with the power transmission.
Hybrid junctions, also known as hybrid couplers, are passive RF devices that may automatically sort and route. An example of a hybrid junction is the Branch Line Coupler, which may have four ports. When a signal is applied at port 1, it is coupled equally to ports 2 and 3 but not to port 4. Simple antennas having multiple ports with hybrid properties may be uncommon.
U.S. Pat. No. 2,147,809, to Alford describes a conjugate bridge circuit providing for isolation between selected ports connected thereto. A 90 and 180 degree phase shifts are used between ports in a transmission line ring, forming a branch line coupler. Radiation from the circuit is not however described.
U.S. Pat. No. 5,977,921 to Niccolai, et al. and entitled “Circular-polarized Two-way Antenna” is directed to an antenna for transmitting and receiving circularly polarized electromagnetic radiation which is configurable to either right-hand or left-hand circular polarization. The antenna has a conductive ground plane and a circular closed conductive loop spaced from the plane, i.e., no discontinuities exist in the circular loop structure. A signal transmission line is electrically coupled to the loop at a first point and a probe is electrically coupled to the loop at a spaced-apart second point. This antenna requires a ground plane and includes a parallel feed structure, such that the RF potentials are applied between the loop and the ground plane. The “loop” and the ground plane are actually dipole half elements to each other.
U.S. Pat. No. 5,838,283 to Nakano and entitled “Loop Antenna for Radiating Circularly Polarized Waves” is directed to a loop antenna for a circularly polarized wave. Driving power fed may be conveyed to a feeding point via an internal coaxial line and a feeder conductor passes through an I-shaped conductor to a C-type loop element disposed in spaced facing relation to a ground plane. By the action of a cutoff part formed on the C-type loop element, the C-type loop element radiates a circularly polarized wave. Dual linear, or dual circular polarization are not however provided.
Although various antennas are known for power transmission and communication they do not include isolated ports and cannot simultaneously provide the radio frequency (RF) power and communications link, e.g. diversity in the field of wireless RF identification (RFID) tags.